1. Field of the Invention
This invention relates to additives to improve the handling of silicone elastomers prepared from polydiorganosiloxanes having Williams plasticity of greater than 0.5 mm and being cured with organic peroxide vulcanizing agents.
2. Description of the Prior Art
Silicone elastomer bases commonly are produced from a polydiorganosiloxane of gum consistency, reinforcing fume silica, an anti-crepe aging material, and many additional additives to modify the properties of the base or the cured silicone elastomer obtained from the base. To achieve desirable physical or electrical properties of the cured silicone elastomer, certain combinations of ingredients are used to prepare the composition of the silicone elastomer base. These bases may not have the most desirable handling properties, that is, it may be softer or tackier than is desired for easy handling or further processing in equipment such as two roll mills, extruders or calenders.
An answer to such problems was disclosed in U.S. Pat. No. 2,721,857 by Dickmann who disclosed the addition of 0.005 to 0.090 parts by weight, based on 100 parts by weight of the organopolysiloxane of, boric acid, boric acid anhydride or alkyl borates. Amounts outside those shown were stated to be inoperative. This boron compound was added to the silicone elastomer stock during its manufacture.
Metevia disclosed in U.S. Pat. No. 3,070,560 that a combination of hydroxylated siloxane and boron in the form of alkyl borates, organosilyl borates, boric acid or boric oxide would improve the handling properties of silicone rubber stock made with the fillers described in U.S. Pat. No. 2,863,846 by Tyler. The hydroxylated siloxane and boron compound were added to the stock during its manufacture. Metevia stated that it was desirable to heat the ingredients of the invention after they have been mixed together at temperatures of 100.degree. to 200.degree. C. for 1 to 3 hours.
Using the technology disclosed by these patents has disclosed problems. The alkyl borates when added to a silicone elastomer base on a mill can give off volatile by-products which present a distinct hazard from fire. When boric acid or boric oxide is used as an additive to a silicone elastomer base on a mill, the reaction rate is so slow that the desired improvement in plasticity and surface tack may not occur for many days.